Heretofore, it has been well known in the orthodontic industry to use orthodontic spreaders or expansion screws for expanding the upper jaw and for correcting the position of maxillary teeth. Examples of such devices are disclosed in U.S. Pat. Nos. 3,284,902; 4,197,644; 4,323,345; 4,324,036; 4,354,832, and 4,482,318. While these devices generally perform their intended function, several difficulties have been encountered with such devices.
One difficulty with these devices is that they require adjustment by inserting a small pin type key into a pin hole of a central spindle or actuating portion at the back of the mouth. The pin is then actuated by rotating it posteriorly through a 90-degree arc and removing it in a direction toward the patient's throat. In addition to the difficulty in inserting the pin in the pin hole, these pins can be quite hazardous because they can easily be dropped in the patient's mouth and swallowed by the patient.
Other difficulties with this pin and spindle adjustment system arise because it only allows a one quarter turn per activation and it is difficult to determine if a constant rate of expansion on both sides of the jaw is provided. Similarly, some of these devices are generally not predictable as to the exact amount of expansion created by each activation.
Prior expansion screws have also included guide pins which take up space and restrict access for ease of adjustability. Also, prior expansion screws, such as the embodiment of FIG. 23 in U.S. Pat. No. 4,323,345 and the devices in U.S. Pat. Nos. 4,324,036 and 4,354,832, relied on swedged parts between a screw and threaded tubing to activate the second stage of expansion, and this arrangement frequently failed, resulting in passive screw rotation without activating the second stage, and an inconsistent expansion rate.
Another problem with these devices is that the width of these devices inhibits their usefulness in the palatal cavity of a narrow mouth such as that of a child who has a severe transverse constriction of the maxillary jaw. This problem is multiplied because the narrower the jaw, the more the expansion is necessary, yet the smaller the space that is available for the spreader doing the expansion.
Other prior expansion screws embodied differing thread pitch on the screws and housing which produced a varied rate of expansion per turn over the entire range of activation. Guide pins are required because of perforations in the central spindle to allow insertion of the activating pin, which weakens the spindle structurally.